The present invention relates to a magneto-rheological (MR) fluid damper, and more particularly, to a linearly-acting MR fluid damper suitable for vibration damping in a vehicle suspension system.
MR fluids are materials that respond to an applied magnetic field with a change in rheological behavior (i.e., change in formation and material flow characteristics). The flow characteristics of these non-Newtonian MR fluids change several orders of magnitude within milliseconds when subjected to a suitable magnetic field. In particular, magnetic particles noncolloidally suspended in fluid align in chain-like structures parallel to the applied magnetic field, changing the shear stress on adjacent shear surfaces.
Devices such as controllable dampers benefit from the controllable shear stress of MR fluid. For example, linearly-acting MR fluid dampers are used in vehicle suspension systems as vibration dampers. At low levels of vehicle vibration, the MR fluid damper lightly damps the vibration, providing a more comfortable ride, by applying a low magnetic field or no magnetic field at all to the MR fluid. At high levels of vehicle vibration, the amount of damping can be selectively increased by applying a stronger magnetic field. The controllable damper lends itself to integration in vehicle suspension systems that respond to vehicle load, road surface condition, and driver preference by adjusting the suspension performance.
MR fluid dampers are based on a piston assembly moving within a damper body tube providing a reservoir of MR fluid. As the piston assembly translates within the damper body tube, MR fluid is allowed to move around or through the piston assembly in a flow gap to the opposite portion of the damper body tube. A magnetic field passing across the flow gap changes the viscosity of the MR fluid in the flow gap. The flow gap thus provides shear surfaces to react to the viscosity of the MR fluid to provide damping.
Increasing the damping performance of the MR fluid damper depends in part upon concentrating the magnetic field at the flow gap. To that end, conventionally, the piston assembly includes a generally cylindrical piston core having an annular recess holding a magnetic coil. The magnetic field from the coil is concentrated at the axially opposing flux pole pieces of a piston core at each end of the flow gap. A magnetic circuit is completed by a magnetic flux return path coupled to each flux pole piece.
Efficiently concentrating the magnetic field at the flow gap requires, in part, an efficient magnetic flux return path. With some MR fluid damper designs, a xe2x80x9csoftxe2x80x9d magnetic material is used to encompass the piston assembly in order to conduct the magnetic field. Low carbon steel is an example of soft magnetic material. One beneficial feature of soft magnetic material is that it conducts magnetic flux better than xe2x80x9chardxe2x80x9d magnetic material.
Conventional MR fluid dampers utilizing soft magnetic material in the magnetic flux return path have various problems. For example, in some MR fluid dampers, a magnetic flux return path is provided by a damper body tube composed of a soft magnetic material such as a low carbon steel. The wall thickness of the damper body tube must be sufficient to avoid magnetic saturation at the higher damping levels. Magnetic saturation occurs when the required damping dictates a magnetic field that exceeds the maximum magnetic field that can be conducted by the wall of the damper tube body. Therefore, greater damping capacity requires a thicker damper tube body wall.
In an MR fluid damper, the damping action occurs by forcing the MR fluid through a flow gap formed between the piston assembly and the wall of the damper body tube. Thus, for a given damper diameter, increasing the wall thickness of the damper body tube reduces the size, and hence, the damping capability, of the piston assembly. Further, the increased amount of steel in the thicker damper body tube increases manufacturing costs and damper weight.
With other MR fluid damper designs, a magnetic flux return path is provided by a ferromagnetic flux ring surrounding the piston core. With these designs, a flow gap passes axially through, rather than around, the piston assembly. Consequently, a relatively thin-walled damper body tube may be made of a material that is not expected to contribute to the magnetic flux return path. Unfortunately, for a given diameter MR fluid damper relying upon a flux ring, the flow gap is moved inward toward the center of the damper body tube, thereby reducing the available shear surface area and hence, the damping capability. MR fluid dampers with flux rings require a structure to hold the flux ring about the piston core. These structures also block part of the available flow path, reducing damping capability. In addition, the cross-sectional area available for the piston core is reduced, decreasing the total amount of magnetic flux that can be conducted around the magnetic circuit, yet further reducing damping capability. As a compromise, some MR fluid dampers use a piston assembly with a thin flux ring, and the magnetic field return path relies on both the thin flux ring and the wall of the damper body tube. Consequently, thin flux ring MR fluid dampers also have problems as do dampers utilizing either a thick flux ring or no flux ring.
Consequently, there is a need for an MR fluid damper with a magnetic field return path that does not saturate with higher damping requirements, does not unnecessarily limit the damping capacity and does not substantially increase the cost or weight of the MR fluid damper.
The present invention provides an MR fluid damper with increased performance. The MR fluid damper of the present invention provides a desired magnetic flux return path without increasing the wall thickness of the damper body tube or changing the location of the flow gap. Thus, the desired magnetic flux return path is provided without adversely influencing the function of any other component of the MR fluid damper or diminishing its damping capacity.
According to the principles of the present invention and in accordance with the described embodiment, the present invention provides a magneto-rheological (xe2x80x9cMRxe2x80x9d) damper having a damper body tube containing an MR fluid. A piston assembly is disposed in the damper body tube and forms an annular flow gap between the piston assembly and the damper body tube. The piston assembly has a piston core containing ferrous material and an electromagnetic coil mounted on the piston core for generating a magnetic field. The damper further includes a ferromagnetic member positioned outside of the damper body tube substantially adjacent the piston assembly for providing at least a part of a magnetic flux return path for the magnetic field. The use of a separate member to provide an additional (or parallel) magnetic flux return path permits increased damping performance without substantially increasing the cost or weight of the MR fluid damper.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.